1. Field of the Invention
The present invention relates to an electrically conductive thermoplastic resin composition, containing metallic powder, suitable for use in the production of electrically conductive resin molded articles as well as electrically conductive coating compositions and electrically conductive adhesive compositions.
The present invention also relates to an electrically conductive coating composition, containing, as a main electrically conductive component, metallic powders in which the electrical conductivity is not decreased for an extended period.
2. Description of the Related Art
Various resin compositions containing the powder of noble metals have heretofore been used as electrically conductive resin molded articles as well as electrically conductive coating compositions and electrically conductive adhesive compositions. Noble metals such as gold and silver are preferable because they are stable against oxidation and afford an excellent electrical conductivity to the compositions containing the same and also retain a stable electrical conductivity for extended periods. Powders of these noble metals, however, are very expensive, and accordingly, when compositions containing a noble metal powder are used as electromagnetic wave shielding molded articles or coating compositions, the production cost becomes high, and, therefore, practical application is difficult. Furthermore, although remarkable developments have been made in various electronic appliances such as computers, video tape recorders, audio appliances, electric appliances, and word processors, these electronic appliances have disadvantages in that erroneous actuation is likely to occur due to electromagnetic waves from the outside, since the electric current used becomes very small because of the use of highly integrated electronic circuits and elements.
Various plastic molded articles are widely used as housings of the above-mentioned electronic appliances because of the need for light weight articles, cost-reductions, and mass production. Plastic molded articles do not provide a shielding effect against electromagnetic waves and, therefore, the electromagnetic wave is transmitted through the plastic molded articles. Thus, the above-mentioned disadvantages cannot be solved in this case.
Various attempts have been made to provide plastic molded articles with shielding effects against electromagnetic waves (i.e., electromagnetic wave shielding properties). For example, electrically conductive coating compositions are coated on the housings of electronic appliances to shield them from electromagnetic waves from the outside and those generated from the appliances themselves. Electrically conductive coating compositions are those containing electrically conductive fine powders dispersed in organic binders. Examples of the electrically conductive fine powders conventionally used are gold, silver, nickel, and copper. Although noble metal powders such as gold and silver have excellent electrical conductivity and are stable against oxidation, and thus retain a stable electrical conductivity for a long time, the powders of these noble metals are very expensive. Accordingly, when noble metallic powders are used as electromagnetic wave shielding coating compositions, the production cost becomes high, and, therefore, a practical application thereof is difficult.
For the above-mentioned reasons, although various attempts have been made, in electrically conductive compositions, to use the powder of inexpensive metals other than the noble metals, such as nickel, copper, iron, and aluminum (i.e., metallic powders), satisfactory results have not been obtained. For example, copper, which is commercially available in large amounts at a cost approximately one hundredth that of silver, the most inexpensive noble metal, has a good electrical conductivity, but copper powder is susceptible to oxidization in coating compositions, and an electrically non-conductive oxide layer will form on the surface of the copper powder, reducing the electrical conductivity, as well as the electromagnetic wave shielding properties, with the lapse of time. Thus, improvements in these properties are desired.
The surface of a commercially available metallic powder is generally covered with an electrically non-conductive oxide layer. Therefore, the desired electrical conductivity cannot be obtained when the commercially available metallic powder is directly incorporated into coating compositions. The oxide layer on the surface of the metallic powder can be conventionally removed by washing the metallic powder with an aqueous acid solution such as hydrochloric acid. But, even when the metallic powder thus washed is mixed with an organic binder to produce a coating composition, the electrical conductivity of the resultant coating composition is lost after the composition is allowed to stand for several days under ambient conditions, although the desired electrical conductivity is exhibited immediately after the formulation or production thereof.